add checks to avoid using optimizations when inappropriate

The fast unpack routines for 3 and 4 channels were not designed to
handle sub-sampled images, but need to appear before other fallback
disablement in the choice logic. Until that choice logic can be
simplified, add additional checks to avoid heading to that branch

Signed-off-by: Kimball Thurston <kdt3rd@gmail.com>

Author: kdt3rd

src/lib/OpenEXRCore/unpack.c

@@ -1437,7 +1437,9 @@ internal_exr_match_decode (
         /* other optimizations would not be difficult, but this will
          * be the common one (where on encode / pack we want to do the
          * opposite) */
-        if (sametype == (int) EXR_PIXEL_HALF &&
+        if (!hassampling &&
+            chanstofill == decode->channel_count &&
+            sametype == (int) EXR_PIXEL_HALF &&
             sameouttype == (int) EXR_PIXEL_FLOAT)
         {
             if (simpinterleave > 0)